1. Field of the Invention
With respect to the field of the invention as established in and by the United States Patent Office, this invention is believed to be found in the general class entitled "Joints and Connections" and in adjustable clamping devices therein.
2. Discussion of the Prior Art
The tool which is the subject of this application is known as a hand clamp. The name probably derives from the fact that it is applied by hand rather than by any power-enhancing means such as air or hydraulic pressure, but it is also true that in most cases a clamp serves as a surrogate human hand, to apply and maintain pressure where and as required. Hand clamps take many forms, some highly specialized; viz. clamps for holding picture frames square for nailing, or for holding two pipe ends in alignment for welding, and as such having little or no other useful purpose. The variety of such special clamps is probably in the several hundreds, many of which have been the subjects of patents.
It is a truism that there is, or can be, no perfect all-purpose clamp. Possible uses are much too varied, and most uses do not require great strength, but rather generous throat depth--and low cost. Thus, in less demanding applications than metal-working, there is a considerable variety of types, each with some actual or perceived advantage over all others.
In metalworking, strength necessary to hold workpieces which are subjected to heavy metal-removal operations is of primary importance. Given this requirement, the number of types currently available is sharply reduced--to three, at most. They are the parallel clamp, the C-clamp, and a proprietary tool known as the Kant-Twist clamp, which was granted U.S. Pat. No. 2,726,694 in the middle 1950's.
Historically, the parallel clamp is the oldest-known form of clamp, being at least 800 years old in wooden form--and still much used in that form by woodworkers. The wooden version is referred to as a hand screw. The version used in metalworking--the parallel clamp--is generally constructed of hardened steel, both the jaws and the spindles.
The C-clamp, in a wooden version, is known to be at least 600 years old. While the evolution of the C-clamp is obscure, most readily available C-clamps today are cast of malleable iron and are not particularly strong. In the metalworking industry the most generally used type of C-clamp is made of forged and heat-treated steel, and is the most durable of all clamps.
The Kant-Twist clamp was designed in recognition of the need for something more versatile than the parallel and C-clamps. After a slow start, it became quite popular. And, after its patent expired circa 1973, copies began to reach these shores from Japan, Taiwan, Korea and, finally, mainland China, so that the American originators have come on hard times.
The parallel clamp is mostly used by tool-and-die makers, instrument makers and other high-precision craftsmen. It serves well only when clamping flat, parallel surfaces, and is only used for that purpose. Carefully used, it will not mar highly-finished surfaces, and it cannot cause workpieces to twist, as can the C-clamp. But its application is time-consuming and requires some degree of skill, since two spindles must be adjusted simultaneously. And the parallel clamp is difficult to use where working space is limited. When it is properly adjusted, its strength is roughly 80% of that of the C-clamp, size for size.
The C-clamp is the most widely used of the three types. It can bring-to-bear the greatest pressure of the three; it is, as has been said, very durable, and it is usually easy to apply. But, in many applications, it has definite faults. The length of the frame plus the length of the spindle, fully extended, is 31/2 to 4 times its clamping capacity. This, in addition to the fact that the T-handle on the end of the spindle must be turned to tighten the clamp, makes it impossible to use in many space-limited situations. Another undesirable feature of the C-clamp is the fact that the spindle is tightened directly against a workpiece, the workpiece being protected by a steel pad attached loosely to the end of the spindle. On a correctly functioning clamp, the pad should remain stationary while the spindle revolves. But this is often not the case: the pad turns with the spindle, causing the clamp to "walk" across a rough surface such as cast iron, or, in the case of smooth workpieces, causing the workpiece under the spindle to twist out of the desired position.
After many centuries of inconvenience, a solution to some of the problems of the parallel and C-clamps was devised--the Kant-Twist clamp. It is more compact and easier to apply than either of the others and, as the name implies, does not twist workpieces or "walk" across rough surfaces. But in strength it is a failure. At best, it can produce about 40% of the pressure of an industrial C-clamp, and some specific designs of the Kant-Twist clamp produce considerably less than 40% of comparable C-clamps. Thus, in the metalworking industry, it must be looked upon as a light-duty clamp. The metalworking industry still needs a better clamp.
Because the industry needs a better clamp, the clamping device of my U.S. Pat. No. 3,736,629 (identified above) was received with great interest by many of the country's most reputable manufacturers. But interest foundered, in every case, on the reef of cost. Several features which were desirable but not necessary could have been simplified, but the heart of the tool--the eccentric cam and its function-limiting means--were too expensive to produce. No inexpensive design could be devised so, after several years of trying, the tool was abandoned. The metalworking industry still needs a better clamp.